The generation of electric power by a nuclear reactor is accomplished by the nuclear fission of radioactive materials. Due to the volatility of the nuclear reaction, nuclear power plants are required by practice to be designed in such a manner that the health and safety of the public is assured.
In conventional nuclear reactors used for generating electric power, the nuclear fuel becomes spent and is removed at periodic intervals from the nuclear reactor and replaced with fresh fuel. The spent fuel generates decay heat and remains radioactive after it has been removed from the nuclear reactor. Thus, a safe storage facility is provided to receive the spent fuel. In nuclear reactors, such as pressurized water reactors, a pool is provided as a storage pool for the spent fuel. The spent fuel pool is designed to contain a level of water such that the spent fuel is stored underwater. The spent fuel pool is typically constructed of concrete and is at least 40 feet deep. In addition to the level of the water being controlled and monitored, the quality of the water is also controlled and monitored to prevent fuel degradation when it is in the spent fuel pool. Further, the water in the spent fuel pool is continuously cooled to remove the heat which is produced by the spent fuel.
In general, a nuclear power plant includes a spent fuel pool cooling system which is designed to remove decay heat generated by stored spent fuel from the water in the spent fuel pool. Removal of the decay heat maintains the spent fuel pool water temperature within acceptable regulatory limits. The spent fuel pool cooling system typically includes a spent fuel pool pump which circulates the high temperature water from within the spent fuel pool through a heat exchanger and then returns the cooled water to the spent fuel pool. In one embodiment, the spent fuel pool cooling system includes two mechanical trains of equipment. Each train includes one spent fuel pool pump, one spent fuel pool heat exchanger, one spent fuel pool demineralizer and one spent fuel pool filter. The two trains of equipment share common suction and discharge headers. In addition, the spent fuel pool cooling system includes the piping, valves and instrumentation necessary for system operation. In this embodiment, one train is continuously cooling and purifying the spent fuel pool while the other train is available for water transfers, in-containment refueling water storage tank purification, or alignment as a backup to the operating train of equipment.
FIG. 1 shows a spent fuel pool cooling (SFPC) system 10 during its normal operation in accordance with the prior art. The SFPC 10 includes a spent fuel pool 15. The spent fuel pool 15 contains a level of water 16 which is at a high temperature as a result of the intense temperature of the spent fuel (not shown) that is transferred from the nuclear reactor (not shown) into the spent fuel pool 15. The SFPC system 10 includes trains A and B. Trains A and B are employed to cool the water in the spent fuel pool 15. As previously described, it is typical to operate either one of train A or train B to continuously cool and purify the spent fuel pool 15 while the other train is available as a back-up. Each of trains A and B include a SFPC pump 25, a heat exchanger 30, and a SFPC demineralizer and filter system 45. These trains share a common suction header 20 and a common discharge header 50. In each of trains A and B, water exits the spent fuel pool 15 through the suction header 20 and is pumped through the SFPC pump 25 to the SFPC heat exchanger 30. In the SFPC heat exchanger 30, a flow line 40 passes water from the component cooling water system (CCWS) (not shown) through the SFPC heat exchanger 30 and back to the CCWS. The heat from the water entering the SFPC heat exchanger 30 (from the spent fuel pool 15) is transferred to the water provided by the flow line 40 and is returned back to the CCWS through the flow line 40. Cooled water exits the SFPC heat exchanger 30 and passes through the SFPC demineralizer and filter system 45 positioned downstream of the SFPC heat exchanger 30. Purified, cooled water exits the demineralizer and filter system 45, is transported through the common discharge header 50, and is returned to the spent fuel pool 15.
Recently, nuclear reactor manufacturers have offered passive plant designs, i.e., plants that will mitigate accident events in a nuclear reactor without operator intervention or off-site power. The Westinghouse Electric Company LLC offers the AP1000 passive plant design. The AP1000 design includes advanced passive safety features and extensive plant simplifications to enhance the safety, construction, operation, and maintenance of the plant. The AP1000 design emphasizes safety features that rely on natural forces. The safety systems in the AP1000 design use natural driving forces such as pressurized gas, gravity flow, natural circulation flow, and convection. The safety systems do not use active components (such as, pumps, fans or diesel generators) and are designed to function without safety grade support systems (such as, AC power, component cooling water, service water, and HVAC). The AP1000 fuel handling area is designed such that the primary means for fuel protection is provided by passive means and relies on the boiling of the spent fuel pool water inventory to remove decay heat. Thus, in extreme cases, the spent fuel pool can boil.
Assuming a complete failure of the active spent fuel pool cooling system, spent fuel cooling can be provided by the heat capacity of the water in the spent fuel pool. Water make-up is provided to the spent fuel pool by a passive means to maintain the pool water level above the spent fuel while boiling of the pool water provides for the removal of decay heat. Boiling of the spent fuel pool water releases large quantities of steam into the fuel handling area. The steam mixes with the air in the fuel handling area and has to be released from this area to prevent a build-up of pressure. The steam/air mixture is released from the fuel handling area into the atmosphere. This can potentially result in the release of radioactive airborne contaminants into the atmosphere.
Analysis has shown that minimal radiation doses that are well within acceptable limits may result from the onset of boiling. However, it is advantageous to provide a spent fuel filtration system and method for further reducing the radioactive doses that are released into the atmosphere from the onset of boiling of the spent fuel pool in the fuel handling area of a nuclear reactor. It is desired that the system and method be a passive mechanism which is simple to design and implement, and is effective to remove radioactive particulates in the event of a spent fuel pool boiling event in the nuclear reactor.